Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X18: Focus Session: Polymer Network Mechanics II |
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Sponsoring Units: DPOLY Chair: Al Crosby, University of Massachusetts, Amherst Room: B117 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X18.00001: Cavitation, Elasticity and Fracture in Strong Hydrogels Jun Cui, Ahmad Madkour, Gregory Tew, Alfred Crosby The interplay between the molecular network and material microstructure of a polymer-based hydrogel is critical for determining both the low strain elastic properties and fracture toughness. We present a novel complex hydrogel network developed by introducing polydimethylsiloxane (PDMS) into a polyethylene glycol (PEG)-based network. Using a combination of conventional characterization techniques, as well as the recently developed technique of cavitation rheology, we investigate the balance of elasticity and fracture in these complex networks. The polymer network maintains elasticity, with negligible hysteresis, at large strains over a wide range of swelling ratios. These properties are investigated across a continuum of length scales ranging from microns to centimeters by taking advantage of cavitation rheology, which uses the onset of an elastic instability to quantify local network mechanics. We compare our results with established scaling theories to describe both the elastic and fracture properties as a function of polymer volume fraction. [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X18.00002: Mechanics of Polymer Networks Subjected to Photochemically-Induced Rearrangement Kevin Long, Timothy Scott, Jerry Qi, Martin Dunn Mechanically responsive, light-activated polymers exhibit complex mechanical behavior in response to light. Recently, several new systems have been developed with unique underlying photomechanical mechanisms. For example, radical-mediated network cleavage, reformation, and consequent rearrangement locally alleviates the macroscopic stress in the material studied here. The state of the art in these systems is confined to simple experiments and demonstrations, and therefore, their use in the scientific and engineering communities is impeded by the lack of theoretical and computational tools to predict their behaviors. We present our efforts to characterize, model, and simulate the continuum-scale photo-mechanical behavior of the network rearranging material. An overview of our multi-physics constitutive model is presented along with companion characterization experiments used in its calibration. A variety of applications are both experimentally and theoretically investigated, and future directions and challenges are presented. [Preview Abstract] |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X18.00003: Simulations of Self-Healing Polymer Networks Arun Kumar N, Evgeny Stukalin, Ludwik Leibler, Michael Rubinstein Self-healing polymeric materials are systems that after damage can revert to their original state with full or partial recovery of mechanical strength. Using hybrid MD/MC simulations we study autonomic self-healing of reversible polymer networks. The self-healing mechanism of dangling chains in polymer networks which can form weak reversible bonds, e.g., hydrogen bond, is based on the slow-dynamics of re-association of the broken reversible bonds. The simulation protocol for self-healing process consists of three steps : (1) equilibration of the reversible network, (2) introduction of a fracture plane in the polymer network across which reversible bonds are not allowed to form, and simultaneous shifting of the fractured sections with respect to each other, while pulling the dangling chains back to their respective halves, and (3) after some waiting time, allowing chain penetration and bonding across fractured interface during the healing time period. The simulation model is used to capture the new features of the self-healing mechanism related to the renormalization of reversible bond lifetime and exchange of bonding partners and is compared to analytical models of this process that account for these features. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:42PM |
X18.00004: Toughening Mechanism of Double Network Hydrogels Invited Speaker: The fundamental toughening mechanism of DN gels [1] is of great interest to researchers. Extensive experimental and theoretical studies have been performed to explain this mechanism [2-7]. Yielding and necking deformation [2] that was observed through tensile tests and rate-independent hysteresis [3] observed through cyclic loading tests have indicated that DN gels can accumulate internal damage before the suffering macroscopic fracture; after damage accumulation, the DN gels become much softer. We assume that on the microscopic level, yielding is caused by the partial breakage and fragmentation of the brittle first network and interconnection among the fragments by the polymer chains of second network [2]. Brown [4] and Tanaka [5] have proposed similar models that can qualitatively explain the anomalously high fracture energy, assuming that the DN gel is locally damaged (yielded) around the crack tip and that the energy dissipated for damage accumulation enhances the effective fracture energy. Using AFM measurements [6] and 3D color laser microscope[7], we successfully detected the existence of softened regions, of several hundreds $\mu $m in thickness, at the crack tip just below the fracture surfaces, which supports the assumption of localized damage accumulation. A linear relationship between the thickness of the softened yielding zone and the fracture energy of the gel was observed, which is in agreement with the local yielding zone explanation. \\[4pt] [1] Gong, J. P.; Katsuyama, Y.; Kurokawa, T.; Osada, Y. Adv. Mater. 2003, 15, 1155. \\[0pt] [2] Na Y.H., Tanaka Y., Kawauchi Y., Furukawa H., Sumiyoshi T., Gong J. P., Osada Y., Macromolecules 2006, 39(14), 4641. \\[0pt] [3] Webber, R. E.; Creton, C.; Brown, H. R.; Gong, J. P. Macromolecules 2007, 40, 2919. \\[0pt] [4] Brown, H. R. Macromolecules 2007, 40, 3815. \\[0pt] [5] Tanaka, Y. Euro Phys. Letter. 2007, 78, 56005. \\[0pt] [6] Tanaka Y.; Kawauchi Y.; Kurokawa T.; Furukawa H.; Okajima T.; Gong J. P. Macrom. Rapid Comm. 2008, 29(18), 1514. \\[0pt] [7] Yu, Q. M.; Tanaka, Y.; Furukawa, H.; Kurokawa, T.; Gong, J. P. Macromolecules, 2009, 42(12), 3852. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 3:54PM |
X18.00005: Self-healing in dually crosslinked nanogels: Computational modeling Solomon Duki, Victor Yashin, German Kolmakov, Anna Balazs We report the results of computational modeling of materials composed of dually crosslinked nanogels. In such materials, permanently crosslinked nanogel particles are bound together through two kinds of crosslinks, namely, the stable and labile ones. Under sufficiently high stress, the strong, stable bonds undergo irreversible rupture, whereas the weak, labile bonds can reform after breakage. We demonstrate that presence of the labile interparticle bonds makes possible the structural rearrangements inside the deformed material. As a result, the catastrophic failure of the material is postponed, and the defects (cavities) in the strained material heal themselves when the stress is released. The mathematical model used in the simulations was developed by us through a bottom-up approach, which allows us to capture the viscoelastic and plastic properties of the material under various deformation regimes. [Preview Abstract] |
Thursday, March 18, 2010 3:54PM - 4:06PM |
X18.00006: Viscosity-Based Constitutive Model for the Nonlinear Deformations of Shape Memory Network Polymers Kristofer Westbrook, Francisco Castro, H. Jerry Qi Shape memory polymers (SMP) are materials that can recover a large pre-deformed shape in response to environmental stimuli. This capability makes SMPs suitable materials for applications such as smart fabrics, biomedical devices and deployable structures. For a thermally induced amorphous SMP, the pre-deformation and recovery of the shape require the SMP to traverse its glass transition temperature (Tg) to complete the shape memory (SM) cycle. The dramatic change in viscosity (molecular chain mobility) as the temperature traverses the Tg is the underlying mechanism of the SM effect. As the temperature decreases from above to below the Tg, the material exhibits a transition from a low to high viscosity and the material relaxation increases substantially. Here, the mechanical response of an acrylate-based polymer network is characterized under various thermomechanical histories. A constitutive model is developed to capture the material behavior and implemented to predict responses of the material in specific biomedical applications. [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X18.00007: Investigation of the Non-Linear Viscoelastic Behavior of Lightly Cross-linked SBR: The Need for a New Modeling Perspective James Caruthers, Aparajita Bhattacharya, Grigori Medvedev An extensive set of both linear and non-linear mechanical experiments including non-linear stress-strain behavior and non-linear creep/recovery has been carried out on a lightly cross-linked SBR. The results have been obtained for a wide range of temperatures, extension rates and stretch ratios. The data set reveals an unexpectedly rich behavior, which cannot be predicted by the traditional constitutive models that are based on an additive combination of hyperelastic and quasi-linear viscoelastic contributions. The inability of traditional constitutive models to describe the data is particularly striking for a high extension rate deformation followed by a slow extension rate (e.g. creep) as contrasted to deformations at slow extension rates. The hyperelastic model of rubber elasticity is shown to provide a satisfactory description of the equilibrium behavior; thus, the results in the current study indicate the need for the development of a new type viscoelastic model for elastomers. Potential candidates for the needed constitutive description will be discussed. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X18.00008: Modeling the BZ reaction in gels with chemo-responsive crosslinks Victor V. Yashin, Olga Kuksenok, Anna C. Balazs We model chemo-responsive polymer gels, which expand and contract periodically in response to the ongoing oscillatory Belousov-Zhabotinsky (BZ) reaction. This behavior is due to a ruthenium catalyst, which is grafted to the polymers and affects the polymer-solvent interactions as it undergoes the redox oscillations in the course of the reaction. We consider a permanently crosslinked polymer gel that encompasses Ru(terpy)$_ {2}$ catalytic units having both the terpyridine ligands chemically bonded to the network. It is known that oxidation of the Ru metal-ion from Ru(II) to Ru(III) results in the dissociation of the Ru(terpy)$_{2}$ complexes since the Ru(III) ions form only mono-complexes with terpyridine. Hence, the grafted Ru(terpy)$_{2}$ units would effectively create crosslinks that break and re-form in the response to the BZ reaction. We modified the Oregonator model for the BZ reaction and took into account that the re-formation of Ru(terpy)$_{2}$ complexes is frustrated by the gel network. The time-dependent elastic contribution of the Ru(terpy)$_{2}$ crosslinks was described by the BKZ-type constitutive equation. We report on the results of simulations in 1D. We show, in particular, that compression of the sample leads to stiffening of the gel due to an increase in the crosslink density. [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X18.00009: Controlling bending of chemo-responsive gels with gradient in cross-link density Olga Kuksenok, Victor V. Yashin, Anna C. Balazs Chemo-responsive gels undergoing the Belousov-Zhabotinsky (BZ) reaction exhibit self-sustained pulsations, which can be harnessed to perform mechanical work. To utilize such BZ gels in a number of technological applications, it is critical to develop a robust approach for controlling their bending and stretching. Using our recently developed gel lattice spring model, we focus on the three-dimensional dynamics of chemo-responsive gels that encompass gradients in their cross-link density. Specifically, we simulate the dynamics of long thin rectangular filaments with a gradient in the cross-link density perpendicular to the long axis. We show that the shape of the sample strongly depends on the physical properties of the gel and the parameters of the BZ reaction. We compare our simulation results on the amplitude of bending with experimental data obtained in Ryo Yoshida's group at the University of Tokyo. We also simulate the dynamics of BZ gels that contain a helical distribution of the gradient in cross-link density and show that such samples form ``springs'' that exhibit complex motion. Finally, we investigate how bending and spring-like motion of the sample is affected when the sample is attached with one of its ends to a substrate (thus forming an autonomously oscillating cilium). Our studies constitute the first simulation studies of the dynamics of three-dimensional heterogeneous chemo-responsive gels. [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X18.00010: Prediction of Gel Modulus using the Gel-Tensile Blob (GTB) Model for Network Structure Gregory Beaucage, Sathish K. Sukumaran Network structure in gels, as determined by small-angle scattering, bears little resemblance to the structure expected from the Flory-Rehner, c* or other models for gel properties. Scattering shows a universal excess in scattering intensity at moderate-q that displays a size larger than that of a chain between network crosslinks. Further, a substructural size, often associated with the chain-coil of the network mesh, deviates from linear chain scattering. In this presentation, recent developments in the understanding of branch topology (Beaucage 2004, Ramachandran 2008,2009) are applied to networks in order to predict mechanical properties, particularly the modulus of swollen networks. The approach is based on quantification of the minimum dimension and connectivity dimension for gels and coupling of these descriptions with the gel-tensile blob (GTB) model previously described by Sukumaran (2001,2005). Beaucage G, \textit{Phys. Rev. E} \textbf{70} 031401 (2004).; Ramachandran R, et al. \textit{Macromolecules} \textbf{41} 9802-9806 (2008).; Ramachandran R, et al. \textit{Macromolecules}, \textbf{42} 4746-4750 (2009).; Sukumaran SK {\&} Beaucage G \textit{Europhys. Lett.} \textbf{59} 714-720 (2002).; Sukumaran SK {\&} Beaucage G, et al. \textit{Eur. Phys. J. E} \textbf{18} 29-36 (2005). [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:06PM |
X18.00011: Polymeric Microgels as Potential Drug Delivery Vesicles Ryan McDonough, Kiril Streletzky, Mekki Bayachou, Pubudu Peiris The temperature dependent volume phase change of cross-linked amphiphilic molecules (microgels) suggests their use as drug delivery vesicles. Drug particles aggregate in the slightly hydrophobic microgel interior. They are stored in equilibrium until the critical temperature (Tv) is reached where the volume phase change limits available space, thus expelling the drugs. This loading property of hydroxypropylcellulose (HPC) microgels was tested using amperometric analytical techniques. Small molecules inside microgels do not approach the electrode surface, which decreases current signal. A room temperature (Troom) flow amperometric measurement comparing microgel/paracetamol solution with control paracetamol samples yielded about 20 percent concentration reduction in the microgel sample. Results from the steady-state electrochemical experiment confirm the 20 percent concentration drop in the microgel sample compared to the control sample at Troom. Using the steady-state experiment with a cyclic temperature ramp from Troom to beyond Tv showed that the paracetamol concentration change between the temperature extremes was greater for the microgels than for the controls. An evolving aspect of the study is the characterization of microgel shrinkage from in situ, temperature controlled liquid AFM images as compared to previously completed DLS characterization of the same microgel sample. [Preview Abstract] |
Thursday, March 18, 2010 5:06PM - 5:18PM |
X18.00012: Effect of heating rate, polymer concentration, and cross-linking density on volume phase transition of microgels Kiril A. Streletzky, John T. McKenna, Imaan Benmerzouga The structure and dynamics of crosslinked hydroxypropylcellulose nanoparticles (microgels) was studied by dynamic light scattering below and above the volume phase transition temperature Tv. Microgels were synthesized at different polymer, salt concentration and varying cross-linking density. The microgel size was found to strongly depend on polymer concentration. The effective cross linking density affected the monodispersity of microgels. Both nearly exponential and highly non-exponential spectra were systematically analyzed by spectral time moment analysis below and above Tv. The angular dependence of the spectra was studied to check the diffusive nature of the observed spectral modes. The analysis below Tv revealed one or two faster modes (depending on synthesis parameters) with diffusive characteristics and apparent radii of 20-30 and 150-650nm and in some cases a slower mode which was independent of the scattering angle and reminiscent of the slow polymer mode observed in identical non-crosslinked solutions. The analysis of the data above Tv yielded strong dependence on the heating rate. One step fast heating resulted in disappearance of the smaller microgel particles and deswelling of large ones down to 80-150nm. Under slow multistep heating both microgel-identified modes remain present while the larger microgels grow in size to 800-900nm. [Preview Abstract] |
Thursday, March 18, 2010 5:18PM - 5:30PM |
X18.00013: Ultraslow Stretching of Polymer Gels in Solvents Kenji Urayama, Akihiro Konda, Toshikazu Takigawa We have investigated the nonlinear stress-strain relations and deformation behavior of a highly swollen polymer gel (with a length of ca. 5 cm and a thickness of ca. 5 mm) in solvents under ultraslow stretching rates (in the order of 0.1 micrometer/sec) that are comparable to or slower than the time scale of the diffusion of the gel. Under conventional tensile speeds, the stress-strain relations are independent of stretching rate and the Poisson's ratio is close to 0.5. In contrast, when the stretching rate is comparable to the diffusion rate of networks, the effects of stretching-induced swelling during elongation become pronounced: The induced swelling causes a reduction in Poisson's ratio as well as a decrease in stress. As a result, we observe a marked dependence of Poisson's ratio and stress-strain relations on stretching speeds in the corresponding time scale. In the extremely slow stretching, the induced swelling is fully equilibrated during elongation: The corresponding Poisson's ratio is ca. 0.25. In such slow strain-rate region, the mechanical properties become independent of strain rate again. [Preview Abstract] |
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